Greece: Has always been a loyal ally of Serbia, they even
refused to take part in 99' NATO bombing of Serbia.
Russia: Biggest ally and friend of Serbia.
Belarus: Can't because of Russia.
Moldova: Cause of Russia and Transnistria.
Romania: Cause of Transylvania.
Bosnia: Cause 1/3 of population are Serbs.
Slovakia is the only one that doesn't have a clear motive or interest. So idk them.